X-ray detectors are used in medical examinations, for example in computed tomography recordings with the aid of x-ray radiation. These x-ray detectors can be embodied as scintillator detectors or detectors with direct converters. An x-ray detector is to be understood below as any type of detector, which detects either x-rays or other hard rays, such as Gamma rays for example.
In a detector made of a scintillator material the scintillator material is excited during the passage of the x-ray radiation and the excitation energy is emitted again in the form of light. This visible light created in the material is measured with the aid of photodiodes. The scintillator detector is embodied as a rule as a type of array comprising a number of scintillator elements, wherein the scintillator elements are assigned individual photodiodes, so that the photodiodes likewise form an array.
Detectors with direct converters on the other hand feature semiconductor materials which undertake a direct conversion into an electrical signal of the radiation striking them. The x-ray radiation striking the detectors directly creates charge carriers in the form of electron-hole pairs. By application of a voltage (bias voltage) to the semiconductor material, the charge carrier pairs are separated by the electrical field created thereby and reach electrical contacts or electrodes, which are attached to the semiconductor material (see FIG. 1). Through this an electrical charge pulse is created, which is proportional to the absorbed energy and is evaluated by downstream readout electronics. Semiconductor detectors employed in the area of human-medical imaging, based on CdTe or CdZnTe for example, have the advantage compared to the scintillator detectors generally used today in this area, that an energy-sorting counting is possible with them, i.e. the detected x-ray quanta can be divided up, as a function of their energy for example, into two classes (high-energy and low-energy) or into a number of classes.
During the operation of semiconducting, direct-converting radiation detectors, such as for example detectors based on CdTe or CZT, the phenomenon of polarization occurs during irradiation by Gamma and x-ray radiation, especially at high intensities. This manifests itself in an unwanted change of the internal electrical field in the semiconductor material of the detector. Because of the polarization, the charge carrier transport characteristics and thereby also the detector characteristics change. In particular the said changes lead to a change of the signal characteristics of the measurement signal as a function of the time. In other words, because of the polarization, the intensity of the measurement signal changes over time, with the radiation dose remaining the same. This phenomenon is also called signal drift. A detector is constructed from a plurality of pixels. Since the signal drift of the individual pixels is different, a distribution of the signal drift factors assigned to the individual pixels exists for the detector. Over time or under irradiation respectively this distribution changes, wherein the breadth of the distribution of the signal drift factors increases much more strongly than the average value of this distribution.
One possibility of reducing the signal drift consists of making use of the fact that the breadth of the distribution of the signal drift factors grows more strongly than the change in the average value of the distribution. In this case a number of detectors are combined into groups of individual pixels, so-called macropixels. These macropixels can comprise a number of 2×2, 3×3 or 4×4 individual pixels for example. In order to reduce the signal drift, individual pixels which are drifting strongly are completely excluded from the signal transmission. In this way an improved drift behavior of the detector signal is achieved. However this improvement comes at the expense of a very great deterioration in the efficiency of the detector, i.e. a signal utilization reduced by for example 6.25% to 25% and thus also a correspondingly worsened signal-to-noise ratio (SNR) or a worsened dose utilization.